The present invention relates generally to the removal and recovery of various heavy metals from precipitated iron-bearing sludges. As used herein, the term "heavy metals" refers to both the cationic and anionic forms of non-ferrous metals and metalloids (e.g., arsenic) which have an atomic number greater than that of calcium.
There is increasing concern over the hazards posed by the rising levels of heavy metals in the world's water supplies. Most heavy metals are toxic to some degree to all life-forms, with aqueous concentrations even as low as 0.05 ppm being hazardous to many aquatic flora and fauna. In humans, toxic heavy metal poisoning can lead to severe nervous system disorders and, in extreme cases, death. Even trace amounts of heavy metals are potentially dangerous, because they do not decompose over time (as do most organic pollutants) and often accumulate within an organism throughout its lifetime. This accumulation effect is accentuated in succeeding species along the food chain.
As a consequence of this problem, "industry" is being forced to abandon many previously used toxic waste water disposal techniques such as deep well injection, and to replace them with treatment processes capable of virtually eliminating heavy metals from aqueous wastes. In many cases, however, this requirement is very difficult to fulfill. The metal finishing industry, for example, employs a variety of processes which, typically, generate large volumes of aqueous waste material. Many of these wastes contain concentrations in excess of 10% of heavy metals such as zinc, nickel, copper, chromium, lead, cadmium, iron, tin, gold, and silver. Since over a billion gallons of such wastes are generated daily by the 8000 or so metals finishing plants operating in the United States, it is obvious that a considerable amount of toxic waste solution is formed.
In response to this, numerous heavy metals removal methods including evaporation/crystallization, absorption, dialysis, electrodialysis, reverse osmosis and ion exchange have been proposed for the metals finishing and plating industries, with varying degrees of success. Another method is to sequester and remove toxic heavy metals in a flocculated coagulant comprised of one or more hydrated oxides of iron. At the present time, however, the most usual method for removing these metals is by alkali precipitation, forming a semi-solid "sludge" of insoluble metal hydroxides, and disposing of this material by depositing it in a certified "toxic waste" landfill.
However, the number of toxic waste landfills is relatively low and the sheer volume of these wastes is rapidly depleting the space available in them. Furthermore, there is considerable resistance in most communities to either creating new landfills or enlarging old ones. This, coupled with the size of the paperwork trail which must be created for each shipment, adds up to a situation wherein the price for disposing these wastes is becoming an ever-increasing fraction of the "cost of doing business." For example, fees, transportation costs, and other expenses for such disposal can easily reach as much as $500/ton of sludge. It would be highly desirable if the amount of sludge could be reduced.